Nuclear reactor plant

ABSTRACT

The invention provides a cooling system which includes at least one and preferably a plurality of coolant chambers arranged around a heat source, typically a nuclear reactor. A coolant inlet pipe enters the or each coolant chamber at a high level and extends downwardly through the coolant chamber to a discharge end positioned at a low level within the coolant chamber. At least one anti-siphon bleed opening is provided in that portion of the coolant pipe which is positioned at the highest level within the coolant chamber.

[0001] THIS INVENTION relates to a nuclear reactor plant. It furtherrelates to a method of operating and to a method of constructing anuclear reactor plant. It also relates to a cooling system.

[0002] In a nuclear reactor plant, use is often made of a liquid coolantsuch as inhibited demineralized water, to cool the reactor and thecavity in which it is installed. Typically, use is made of a closed loopcooling system which includes one or more coolant chambers, arrangedaround at least part of the reactor, and pump means for pumping thecoolant into and through the coolant chambers. A coolant inlet typicallyleads into the coolant chambers at a low level and a coolant outletleads from the coolant chambers at a high level.

[0003] A problem with this arrangement is that, should a breach occur inthe inlet pipe, the coolant will drain from the coolant chambers whichcould lead to a potentially dangerous situation arising.

[0004] It is an object of this invention to provide means which theInventors believe will at least alleviate this problem.

[0005] According to the invention there is provided a nuclear reactorplant which includes

[0006] a heat source;

[0007] at least one coolant chamber positioned in proximity to the heatsource;

[0008] a coolant inlet pipe which enters the coolant chamber at a highlevel and extends downwardly through the coolant chamber to a dischargeend positioned at a low level within the coolant chamber; and

[0009] an outlet leading from the coolant chamber at a high level.

[0010] The nuclear reactor plant may include a plurality of coolantchambers arranged around the heat source, each of at least some of thecoolant chambers having an inlet pipe which enters the coolant chamberat a high level and extends downwardly through the coolant chamber to adischarge end positioned at a low level within the coolant chamber.

[0011] In one embodiment of the invention the heat source is a nuclearreactor. Preferably, the reactor may be a high temperature gas-cooledreactor of the type known as a Pebble Bed Reactor in which fuel,comprising a plurality of generally spherical fuel elements, is used.The fuel elements may comprise spheres of fissionable material in aceramic matrix, or encapsulated in the ceramic material. In thisembodiment of the invention gas coolant, eg helium, is fed through thereactor and liquid coolant is fed through the or each coolant chamber.

[0012] In another embodiment of the invention the heat source is a usedfuel storage facility.

[0013] The plant may include anti-siphon means to reduce the risk thatcoolant will be siphoned from the coolant chamber, eg as a result of abreach occurring in the inlet pipe outside the coolant chamber.

[0014] The anti-siphon means may include an anti-siphon valve mounted inthe inlet pipe, typically at the highest point thereof.

[0015] Instead, or in addition, the anti-siphon means may include atleast one anti-siphon bleed opening provided in that portion of thecoolant inlet pipe positioned within the coolant chamber at a positionspaced from the discharge end whereby the coolant inlet pipe and thecoolant chamber are connected or connectable in flow communication.

[0016] Preferably, a plurality of anti-siphon bleed openings is providedin that portion of the coolant inlet pipe which is positioned at thehighest level within the coolant chamber.

[0017] The anti-siphon bleed openings may be in the form of holes in thepipe dimensioned to be sufficiently small so that, in normal use, thesmall amount of coolant flowing therethrough into the coolant chamberwill have no or little detrimental effect on the cooling system andsufficiently large such that in the event of coolant being siphoned fromthe coolant chamber, when the coolant level in the coolant chamber fallsbelow the level of the holes, sufficient gas, typically air, will bedrawn from the coolant chamber into the coolant inlet pipe to break thevacuum and stop the siphoning.

[0018] The anti-siphon bleed opening may be in the form of holes in thecoolant inlet pipe which will have a combined area of between 1% and 10%of the cross-sectional area of the coolant inlet pipe.

[0019] In an inlet pipe having a nominal diameter of 100 mm, typicallybetween 4 and 8 anti-siphon bleed openings will be provided. The bleedopenings will typically be circular and have a diameter of between 5 and10 mm.

[0020] The plant typically includes a pump, an outlet of which isconnected to the or each coolant inlet pipe. The pump and the or eachcoolant chamber typically form part of a closed loop cooling system.

[0021] The invention extends to a method of operating a nuclear planthaving a heat source and at least one coolant chamber positioned inproximity to the heat source which method includes the steps of

[0022] feeding coolant into the coolant chamber through a coolant inletpipe which enters the coolant chamber at a high level and extendsdownwardly through the coolant chamber to discharge coolant into thecoolant chamber at a low level; and

[0023] removing coolant from the coolant chamber at a high level.

[0024] The method may include inhibiting the draining of coolant fromthe coolant chamber by being siphoned from the coolant chamber throughthe coolant inlet pipe.

[0025] The method may include the step of, in the event of coolant beingsiphoned from the coolant chamber through the coolant inlet pipe,bleeding gas into the coolant inlet pipe to stop the siphoning.

[0026] The method may include bleeding gas from the coolant chamberthrough at least one bleed opening in the coolant inlet pipe into thecoolant inlet pipe when the level of liquid coolant in the coolantchamber falls below the level of the at least one bleed opening.

[0027] The invention further extends to a method of constructing anuclear reactor plant having a reactor cavity and at least one coolantchamber positioned in proximity to the reactor cavity which methodincludes providing, a coolant inlet pipe which leads into the at leastone coolant chamber at a high level and extends downwardly through thecoolant chamber to a discharge position at a low level of the coolantchamber.

[0028] The method may include the step of providing a plurality ofcoolant chambers around the reactor cavity.

[0029] The method may include providing anti-siphon means in the inletpipe.

[0030] It will be appreciated that whilst the primary application of theinvention is in respect of a nuclear reactor plant, the cooling systemdescribed may well have other applications.

[0031] Hence, the invention extends to a cooling system which includes

[0032] at least one coolant chamber;

[0033] a coolant inlet pipe which enters the coolant chamber at a highlevel and extends downwardly through the coolant chamber to a dischargeend positioned at a low level within the coolant chamber; and

[0034] an outlet leading from the coolant chamber at a high level.

[0035] The cooling system may include a plurality of coolant chambersarranged around a heat source, each of at least some of the coolantchambers having an inlet pipe which enters the coolant chamber at a highlevel and extends downwardly through the coolant chamber to a dischargeend positioned at a low level within the coolant chamber.

[0036] An advantage with this arrangement is that, should a breach inthe inlet pipe occur, the coolant will not simply drain from the coolantchamber.

[0037] The invention will now be described, by way of example, withreference to the accompanying diagrammatic drawings.

[0038] In the drawings,

[0039]FIG. 1 shows a schematic layout of part of a cooling system for anuclear reactor plant; and

[0040]FIG. 2 shows, on an enlarged scale, part of a coolant inlet pipeof the cooling system.

[0041] In the drawings, reference numeral 10 refers generally to part ofa cooling system of a nuclear reactor plant in accordance with theinvention. In the embodiment shown the cooling system is used to coolthe nuclear reactor part of which is generally indicated by referencenumeral 11. It may however also be used for cooling a used fuel storagefacility.

[0042] The nuclear reactor 11 is positioned in a cavity defined within aconcrete shell (not shown) and is at least partially surrounded by aplurality of coolant chambers 12, one of which is shown in the drawings.Each chamber 12 is defined by a circular cylindrical wall 13, typicallyin the form of a length of pipe, a top 14 and a bottom 15 sealing offthe ends of the wall 13.

[0043] The cooling system 10 includes a pump 16 having a suction orinlet side 18 and a discharge or outlet side 20.

[0044] A coolant inlet pipe 22 is connected to the outlet 20 of the pump16 and extends downwardly through the top 14 of the vessel to the bottomof the coolant chamber 12 at which it terminates in a upwardly directeddischarge end 24. An outlet 26 leads from the vessel at a high level andis connected via piping 28 and other cooling circuit elements, generallyindicated by reference numeral 30 to the inlet 18 of the pump 16.

[0045] Hence, the cooling system is a closed loop cooling system.

[0046] As can best be seen in FIG. 2 of the drawings, a plurality ofanti-siphon bleed openings in the form of holes 32 provided in thehighest portion of the coolant inlet pipe 22 positioned within thecoolant chamber 12.

[0047] In use, the pump 16 pumps coolant, typically in the form ofinhibited demineralised water through the coolant inlet pipe 22 where itis discharged into each of the coolant chambers 12 at a low levelthrough the discharge end 24 of the associated coolant inlet pipe 22.

[0048] The coolant flows upwardly through the coolant chamber 12extracting heat from the reactor and the reactor cavity and the heatedcoolant flows from the coolant chamber 12 through the pipe 28 where itis cooled and recycled.

[0049] In the event of a breach or rupture in the coolant inlet pipe 22the possibility exists that, depending upon the position of the breach,coolant will be siphoned from the coolant chamber 12 through the coolantinlet pipe 22. However, as the level of coolant in the coolant chamber12 falls below the level of the holes 32, air from the coolant chamber12 will flow into the coolant inlet pipe 22 thereby breaking the vacuumand stopping the siphoning to ensure that a relatively high level ofcoolant remains within the coolant chamber 12.

[0050] The reactor can then be shut down, if necessary, and remedialaction taken e.g. by repairing the breach.

[0051] The holes 32 are typically dimensioned so that in normal use,coolant being pumped by the pump 16 which leaks through the holes 32into the coolant chamber 12 will have no or little detrimental effect onthe cooling system. However, the holes are sufficiently large to bleedenough air into the coolant inlet pipe 22 to break the vacuum and stopthe siphoning process. Naturally, the dimensions may vary depending uponthe intended application. However, the Inventors believe that in aninlet pipe 22 having a nominal diameter of 100 mm, typically between 4and 8 holes of between 5 and 10 mm diameter will be provided.

[0052] If desired, an anti-siphon valve 34 can be mounted in the inletpipe 22. The anti-siphon valve 34 is typically positioned in the pipingnetwork at the highest point. The anti-siphon valve is configured toopen when the pressure in the affected pipe drops below atmosphericpressure thereby permitting air to enter the affected pipe, equalisingthe pressure and stopping the siphoning action.

[0053] The Inventors believe that by leading the inlet pipe into thecoolant chamber from a high level, the risk that the coolant chamberwill be drained as a result of a breach in the inlet pipe is reducedthereby substantially enhancing the safety of a nuclear reactor plant ofwhich the cooling system forms part. Further, the provision of theanti-siphon means in the form of the bleed openings 32 and valve 34serves to reduce the risk that coolant will be lost from the coolantchamber as a result of siphoning. The Inventors believe that, inparticular, the provision of the anti-siphon bleed openings will providea simple, reliable and cost effective method of reducing the risk ofcoolant being lost from the coolant chamber as a result of siphoning.

1. A cooling system which includes at least one coolant chamber; acoolant inlet pipe which enters the coolant chamber and extendsdownwardly through the coolant chamber to a discharge end positionedwithin the coolant chamber; an outlet leading from the coolant chamber;and at least one anti-siphon bleed opening provided in that portion ofthe coolant inlet pipe positioned in the coolant chamber at a positionspaced from the discharge end whereby the coolant inlet pipe and thecoolant chamber are connected or connectable in flow communication.
 2. Acooling system as claimed in claim 1, which includes a plurality ofcoolant chambers arranged around a heat source, each of at least some ofthe coolant chambers having an inlet pipe which enters the coolantchamber and extends downwardly through the coolant chamber to adischarge end positioned within the coolant chamber and which has atleast one anti-siphon bleed opening therein.
 3. A nuclear reactor plantwhich includes a heat source; and a cooling system as claimed in claim 1or claim 2 for cooling the heat source.
 4. A nuclear reactor plant asclaimed in claim 3, in which the heat source is a nuclear reactor.
 5. Anuclear reactor plant as claimed in claim 3, in which the heat source isa used fuel storage facility.
 6. A nuclear reactor plant as claimed inany one of claims 3 to 5, inclusive, which includes an anti-siphon valvemounted in the inlet pipe.
 7. A nuclear reactor plant as claimed in anyone of claims 3 to 6, inclusive, in which a plurality of anti-siphonbleed openings is provided in that portion of the coolant inlet pipe,which is positioned at the highest level within the coolant chamber. 8.A nuclear reactor plant as claimed in claim 7, in which the anti-siphonbleed openings are in the form of holes in the coolant inlet pipe whichwill have a combined area of between 1% and 10% of the cross-sectionalarea of the coolant inlet pipe.
 9. A nuclear reactor plant as claimed inclaim 7 or claim 8, in which the inlet pipe has a nominal diameter of100 mm and between four and eight anti-siphon bleed openings areprovided.
 10. A nuclear reactor plant as claimed in any one of claims 7to 9, inclusive, in which the bleed openings are circular and have adiameter of between 5 and 10 mm.
 11. A nuclear reactor plant as claimedin any one of the preceding claims, which includes a pump, an outlet ofwhich is connected to the or each coolant inlet pipe.
 12. A method ofoperating a nuclear plant having a heat source and at least one coolantchamber positioned in proximity to the heat source which method includesthe steps of feeding coolant into the coolant chamber through a coolantinlet pipe which enters the coolant chamber and extends downwardlythrough the coolant chamber to a discharge end from which coolant isdischarged into the coolant chamber at a level which is lower than thelevel at which the inlet pipe enters the coolant chamber; removingcoolant from the coolant chamber at a level which is above the level ofthe discharge end of the coolant inlet pipe; and inhibiting the drainingof coolant from the coolant chamber by being siphoned from the coolantchamber through the coolant inlet pipe.
 13. A method as claimed in claim12, which includes the step of, in the event of coolant being siphonedfrom the cooling chamber through the coolant inlet pipe, bleeding gasinto the coolant inlet pipe to stop the siphoning.
 14. A method asclaimed in claim 13, which includes bleeding the gas from the coolantchamber through at least one bleed opening in the coolant inlet pipeinto the coolant inlet pipe when the level of coolant in the coolantchamber falls below the level of the at least one bleed opening.
 15. Amethod of constructing a nuclear reactor plant having a reactor cavitywhich includes the steps of providing a plurality of coolant chambersaround and in proximity to the reactor cavity; and in each of at leastsome of the coolant cavities, providing a coolant inlet pipe which leadsinto the coolant chamber at a high level at or adjacent the top thereofand extends downwardly through the coolant chamber to a dischargeposition at a level within the coolant chamber which is lower than thelevel at which the inlet pipe leads into the coolant chamber, eachcoolant inlet pipe having at least one anti-siphon bleed openingprovided in that portion of the coolant inlet pipe positioned in thecoolant chamber.
 16. A cooling system as claimed in claim 1,substantially as described and illustrated herein.
 17. A nuclear reactorplant as claimed in claim 3, substantially as described and illustratedherein.
 18. A method of operating a nuclear plant as claimed in claim 12substantially as described and illustrated herein.
 19. A method ofconstructing a nuclear reactor plant as claimed in claim 15substantially as described herein.
 20. A new plant, method and systemsubstantially as described herein.